Means of compensating for drift of photoelectric devices and associated circuits



April 18, 1944. n WOLOSCHAK MEANS OF GOMPENSATIHG FOR DRIFT OFPliCHl'OIEiLnEGERIC DEVICES AND ASSOCIATED CIRCUITS Filed Sept. 1-1

1942 5 Sheets-Shani 1 )umvme. H ffonsfanfin mamm- April 18, 1944.

5 Shuts-Shut 2 K. WOLOSCHAK MEANS OF COMPENSATING FOR DRIFT OFPHOTOELECI'RIC DEVICES AND ASSOCIATED CIRCUITS Filed Sept. 1'7. 1942 2vi -Eon IN VEN TOR.

l" M M M m i m M V Q "mu Ai-gys.

Aim-i118, 1944.

K. WOLOSCHAK 2,347,015

MEANS OF COMPENSATING FOR DRIFT OE PHOTOELECTRIC DEVICES AND ASSOCIATEDCIRCUITS Filed Sept. 17 1942 5 Sheets-Sheet 3 JNVENTOR. KonafanhnWa/osoha Affyli 8 1944- K. WOLOSCHAK 2,3 7,0 5

MEANS OF COMPENSATING FOR DRIFT 0F IHOTOELECTRIC DEVICES AND ASSOCIATEDCIRCUITS Filed Sept. 17, 1942 5 Sheets-Sheet 4 x INVENTOR. E 2Kansfanf-m wolaaohak Q 5 BY E u: a i

18,1944. K. WOLOSCHAK 2,347,015

MEANS OF COMPENSATING FOR DRIFT 0F PHO'I'OELECTRIC DEVICES ANDASSOCIATED CIRCUITS Filed Sept. 17, 1942 5 Sheets-Sheet 5 'Aw 's.

Patented Apr. 18, 1944 i 7, OF COMBATING FOR DRIFT F 'PHO'IOELEGTBIODEVICES AND ASSOCI- Konstantin we... rm Chester, s. m, as-

signer to Press Association, Inc, New York. N. Y., a corporation of NewYork Application September 17. 1942, Serial No. 458,842

14 Claims. (Cl. 178-6.!)

} This invention relates to means for compensating for the gradual.drift of the signal output voltage of an electron amplifleror aphotoelectric cell, and the gradual change in the overall gainoftransmission circuits." In accordance with some applications of thisinvention com'- pensation for the drift of. either transmissioncircuits, the photoelectric device or its associated circuits, or thetwo in combination, may be accomplished.

A specific oblect of this invention is the ap plication thereof for usewith facsimile picture transmission systems.

The detailed objects of the invention and details of various circuitarrangements for accomplishing those objects are set forth below in cornnection with the attached drawings in which the same or similarreference numerals are used throughout to indicate the same or similarparts.

This invention resides substantially in the combination, construction,arrangement, relative system.

Figure 2 is a left hand end elevational view of the rotatabiecylindrical support for the picture to be transmitted.

Figure 3 is a schematic circuit arrangement showing amodinc'ation whichmay be applied to the circuit arrangement of Figure i.

Figure 4 is a diagrammatic view of a modified form of the portion of theapparatus of Figure 1 involving the picture support and scanning head.

Figure 5 is a left hand end elevational view of the rotatable picturesupport of the structure of Figure 4.

l lgureoisaviewsimilartoi'iguredofafurther modified construction.

Figure 7 is a complete diagrammatic and circuit schematic arrangementinvolving a still further modified form of transmission equipment ofthis type embodyin the invention with further modifications.

Figure ii is a diagrammatic illustration of the application of thisinvention in the correction of gradual variations in a, transmissionline or other. communication channel.

Figure 9 is a diagrammaticillustraticn oi the application of thesubicctmatter of this invention to a system wherein the gradual drlitlnthe signal output voltage oi a photoelectric device and the gradualvarlationsin the connected transmission line or other communicationchannel are compensated for.

In the use of photoelectric cells and electron multipliers of thephotoelectric type. as for example when used in connection withfacsimile picture transmission systems, there is inherent therein, aswell as in the circuits associated therewith, a tendency for the signalv itage to change gradually due to the drifting of he output voltage ofthe photoelectric devices and to a gradual over-all change or drift inthe gain 0! the circuits associated therewith. It is the prime object ofthis invention to provide means for automatically compensatingior eitherof these drift components, or for both of them.

In accordance with this invention the voltage drift of the photoelectricdevices and associated equipment at the transmitting station may becompensated for by association of the herein disclosed invention withthe equipment at the transmitter. Similar results can be secured by theappropriate application of this invention to the transmitter andreceiver to accomplish correction of the gradual variations in thetransmission line or other communication channel therebetween. As afinalapplicatlon of this invention, portions thereof may be associatedwith the transmitting and receiving equipment to compensate both for thedrift of the photoelectric devices and for the associated andinterconnecting circuits. These various applications oi the inventionwill be separately described hereinafter.

Referring to Figure l of the drawings, a rotatable cylinder I of thetype usually employed in iacsimile picture transmission is diarammatically illustrated to include a shaft 8 for supporting it forrotation on its longitudinal central axis. The picture such as aphotograph to be transmitted, is diagrammatically illustrated at 2applied around the periphery of the cylinder 1 and attached thereto atits top and bottom edges by means of a wedge bar I which clamps the topand bottom edges of the photograph into a longitudinal slot in thecylinder i and holds the photograph in intimate contact with thecylinder l throughout its area. Clamping bars of this type are commonlyused in such systems. As a part of this invention a special contact 5 ismounted on the end of the cylinder I and aligned with the end of theclampi'i bar 4 in a radial direction (or cooperation with contacts whichwill be referred to later.

Only so much of the scanning head has been diagrammatically illustratedherein as is necessary to an understanding of the invention. The partsillustrated are enclosed within a dash line box for the purpose ofindicating that the scanning head 8 is a structural unit which. as iswell known in this art. is driven in synchronism with the rotation ofthe cylinder I. so that it slowly moves longitudinally thereof. Asillustrated, this to unit includes a light source I the light from whichis focused by a lens system 8 to a point P on the surface of thephotograph 2 along an axis at an angle so that the light reflected fromthe point P is refocused by a lens system 8 onto the pickupphotoelectric cell It or electron multiplier having a photoelectriccathode.

with this arrangement, as is well known, when the transmitter is inoperation the point P has a helical path of relative travel with respectto the photograph I. so that the point gradually traverses the entiresurface of the photograph 2 and thereby scans each elemental areathereof. The output of the photoelectric device it varies in accordancewith the reflective characteristics of the elemental areas scanned. Theoutput of the photoelectric device It has been diagrammaticallyillustrated as being supplied by means of the circuit wires II to anamplifier l2, preferably of the vacuum tube type. and associatedequipment. Circuits of this type are well known in the prior art and oneform of this circuit is illustrated in the co-pending application ofJames C. Barnes. Serial No. 426,813, filed January 15, 1942.

The amplified signal currents are supplied from the amplifier I: bycircuit wires I3 and II to the primary of the input transformer ll ofthe multistage vacuum tube signal amplifier, as illustrated.

The secondary of transformer II is connected in the input circuit of atriode vacuum amplifier ill whose output circuit is connected by thecoupling transformer 11 to the input circuit of a second triode vacuumtube amplifier l8. As a further aid in the application of this inventionit maybe noted that these triodes may be the commercially known 7H7tubes connected so as to be used as triodes. The output circuit of thisampliher is connected by the coupling transformer it to the ton line.Anode voltage supply is connected to the lead 28 in accordance with wellknown practice.

In accordance with this invention, the compensating device is associatedwith this amplifier in the following manner. The output leads I! of theamplifier II are also connected to the primary of a coupling transformer20 whose secondary is connected in the input circuit of a diode vacuumtube detector It. This input circuit includes a pair of contacts 22arranged to be interconnected by means of the contact I mounted on thecylinder l. n Contacts 22 are connected by the wires Ii in the inputcircuit of the diode. The output circuit of the diode includes a network24 comprisingaoondensercandaresistorlt. This network 23 has a relativelyhigh time constant due to the fact that the capacitance and theresistance of which it is composed are large.

(:ommon lead 2| extends from the network It to the cathodes of thetriodes II and It. An adjustable connection 26 from the resistor R isconnected to the input circuits of the triodes it and "through theresistors 26 and fl. By-pass condensers are connected between the pointsof connection of the cathodes of the triodes it and I I to the lead 2!and the points of connection of the resistors 20 and 21 to the inputcircuits of those triodes.

In the operation of this system it will be seen that the input circuitto the diode 23 will be completed once for each-revolution of thecylinder 1.

that is. at the time the contacts 22 are interconnected by the contactI. At this time the light point P will be projected pon the surface ofthe clamping bar I. This clamping bar should be made to have uniformreflective characteristics and should preferably be white although agray or other tonal value would be suitable. It thus provides areflection standard of uniform reflecting power throughout its length sothat the light reaching the photoelectric device I I provides a standardreference signal. Since the contact I and strip I are radially aligned,the signal voltage output of the electron device HI is a maximum oncefor each revolution of the cylinder i at the time that the input circuitfor the diode is complete at the contact 8.

At this time the voltage developed across the resistor R is determinedsolely by the signal voltage generated by the light reflected from thestrip 4. If the output signal voltage of the photoelectric device ltdrifts or changes gradually, due to the drifting of the operatingvoltage of the electron device, or to a gradual change in the over-allcircuits connected between it and the detector 28, the voltage acrossthe resistor R'will likewise change slowly. -This rectified voltageacross the resistor R can be used as illustrated inFigureltovarythegridbiasvoltageofoneor more of the trlodcs of theamplifier (in the case illustrated this bias control is shown for bothstages of the amplifier) to compensate for the original signal voltagechange resulting from the drift components mentioned. The average valueof this compensating voltage can be controlled by adjustment of theconnection of line 26 to the resistor R as illustrated.

In order to increase the sensitivity of this arrangement, themodification of Figure 3 may be employed, which consists solely ofinserting in the connection between the resistor R. and the lead a abattery or other source of constant voltage 8|! so that it is inopposition in polarity to the compensating bias voltage generated in theresistor R.

The voltage value of the' source Ill should be almost equal to thecompensating bias voltage drop in the resistor It. From an examinationof Figure 3 it will be obvious by reason of the use of the samereference numerals how to apply thefeatureoi.l 'igureiltothecircuitoi'illirurel.

By this use of the biasing voltage source ll it will be apparent tothose skilled in the art that the effect of the voltage variationsacross the network Rwillbemoredlrectlyproportionaltothevarlations intheenergy l t from the circuit II thaninthecaseoftheofllgure 1wherethesourc lliisnotemployed.Thereasonsforthlssreatonceapparenttothoseskilled incirouit operations.

Themangementofi 'lguresiandficontainsa variation of a portion of thevention heretofore disclosed wherein theuse of th ping-bar I iswithasasourceofmaximum reflection and a special white target, moving intimed relation with the cylinder, is employed. In this case the lightfrom the source I is focused upon a white target it which is connectedto a shaft on which is mounted a bevel gear ii for rotation therewith.The bevel gear If in turn meshes with a bevel gear I! which is spllnedto the shaft 83 so that itsmay progress therealong as it revalves withthe shaft. The target and gears are mountedonandforma partofthescannlnghead i so that they may move with the other parts of the scanning headlongitudinally of the cylinderiinaccordancewlthcommonpraetioe. Shaft Itisprovided with a gear It which meshes with a gear ill mounted upon theshaft 3 for retation with it and the cylinder. These variouspartsareproportionedsothatssthepowerdrlvterconnec'ting the transmitterand receivr of such a system. Thu is a good point to note. that theapplication of this invention to facsimile picture transmission systemsdoes not represent the only use to which the subject matter of theinvention may be put. Particularly it is to be noted in connection withthe application of the invention as illustrated by Figure 8, that thesubiect matter of this invention may be used for improving transmissionline characteristics generally. Only sufiicient of the transmitter andreceiver of a facsimile picture system is illustrated in Figure 8 tomake the subject matter of the invention clear. The transmittingcylindo:- or support I, rotatably mounted on the shaft 3 and providedwith the cam 5', is like that previously described in connection. forexample, with the description of Figure 1. In this arrangement, however,the receiving support for the receiving negative is illustratedaslikewise comprising a rotatable support It! provided with a shaft uponwhich a cam I similar to the cam 5' is mounted. The scanning andrecording heads have not been shown as unnecessary to an understandingof the invention. The signal to be transmitted, such as the picturesignal, is fed by the circuit it into the transformer 42 and may comedirectly'from the scanning head or an amplifier fed by the scanninghead. The cam 6 operates a single pole double throw switch 4! whosemovable blade is operated by the cam. One of the fixed contacts thereofis connected to one terminal of the secondary of transformer 42. Theother terminal of this secondary is connected to one terminal of thesecondary of a transformer 45 and to one of the transmission circuitlines 48. The movable contact of switch ll is connected to the othertransmission circuit line 8. The other terminal of the secondary oftransformer i is connected to the other fixed contact of switch 43. Theprimary of the transformer ii is fed by a suitable signal source ll,preferably an alternating current generator of constant amplitude. Thebroken line box 41 represents the transmission line or communicationchannel between the transmitter and the receiver with its usual adjunctsand in the various known suitable forms.

The incoming signal from the transmission line is fed to a transformerl3 whose secondary is connected at one terminal to the movable contactof a switch ll operated by the cam 4| on the receiver. Hie fixed contactof this switch is connected to the cathode oi a diode rectifier 51. Theplate circuit of the diode is connected to the other terminal of thesecondary of transformer {I through a network Ii comprising apotentiometer shunted by a capacitor. shunted across the incoming end ofthe transmission line is a transformer 52 whose secondary is connectedto the input of a triode 55 forming part of the compensating amplifierindicated by the broken line box. This input circuit includes the bypasscondenser 58. The output circuit of the triode It feeds a transformer 66whose secondary is connected in a known manner, not shown, to therdcordinghead of the receiver of which the cylinder II is a pert. Thesignal current in this secondary circuit represents the compensatedsignal from the transmitter, that is. the signal from the transmittercompensates for any variations introduced by changes in the transmissioneinciency of the channel 01. This compensation is effected by reason ofthe connections i'romthenetwortli ecroestheterminaleofthe condenser IIin the input of the triode ll. Theresistor-flisprovidedfortheobviouspurpose of creating the proper circuitconditions in conjunction with the adjustment of the potentiometer ofthe network ll.

It will be seen that the compensating amplifier audits manner ofconnection with the diode rectifier and other portions of the circuit issimilar to that previously described and only difiers in that thecompensating amplifier is shown as having one stage, it beingunderstood, of course. that the required number of stages may beemployed as previously indicated in connection with Figure 1. It is, ofcourse, apparent that the operation of this system depends upon suitablesynchronism, the requirement for which is indicated by the relativepositions of the cams 5' and ti. In other words. when the referencesignal source 44 is in circuit at switch 43. switch 4! must be closed sothat the compensating signal circuit is in operation.

In the actual application of this invention these conditions willpreferably exist at the interval between scanning strokes. For examplein the case of the rotatable support type of facsimile system, theseconditions would be maintained at the time the scanning head and recording heads are aligned with the clamping bars of the supports. In thecase of the reciproeating type of supports, this condition would existduring the return stroke of the supports at which time scanning andrebordingnoes not take place. To summarise. the receiving end of theline is periodically connected to the rectifier ,circuit for controllingthe gain of the amplifier at the time the reference signal source isconnected in circuit. all of which shouldoccur at the interval betweenwhat may be termed the scanning strokes. This application of theinvention will be seen to differ from those previously described in thatthe reference signal is. in the former case. produced by a separatereierence signal generator ll as distinguished from the case of thelatter where this signalis gotten 4:. by the use of a target in one formor another of uniform reflection characteristics. p

The arrangement of Figure 9 will be described,

wherein is mustrated the application of this invention to a facsimilepicture transmission eye Mteminordertocompensatenotonlyi'oradrlft m use.'lhepositiveorotherformofscenelto in the signal output voltage of theelectronic device. but also for variations in the transmissionefilcieney of the channel connecting the transmitter to the receiver.

1n the system illustrated diagrammatically in Figure 9, both thetransmitter and receiver have been shown. As illustrated. thetransmitter includes a suitable support such as a drum I,mountedupontheshafttsoastoberotatablein transmittedismountedonthedrum Iand held thereon by meansof the clamping bar reflected from thephotograph is a focusing lens system 9 arranged to focus the reflectedlight on a suitable photoelectric device III. The scanning head unit 8of course movesin timed relation with the support I so as to slowlytravel in an axial direction from one end to the other during scanning.

At this point it may be noted, as those skilled in the art willappreciate. that the subject matter of this invention in all of itsvarious forms, may be equally well applied to a facsimile picturetransmission system where the scene to be transmitted is in the form ofa transparency and the scanning head of a type employing 30 makes onecomplete revolution for each revolution of the drum I and hence isinterposed between the light source I and photoelectric cell III oncefor each rotation thereof.

The output of the photoelectric cell is applied by the circuit wires IIto the input of a transmission amplifler I2 whose output terminals areconnected to the transmission line channel or network 41 connecting thetransmitting station with the receiving station. In this system thesignal voltage is amplified and compensation is applied to the amplifierat the receiving station. The channel I1 is coupled to the signalamplifier by means of aztransformer I5 which corresponds to thesimilarly numbered transformer at the transmitter in the system ofFigure l. A multistage signal amplifier is employed, as before,including the trlode amplifiers I8 and Ill. The compensating detector inthe form of a diode 23 has its input circuit coupled to the receivingstation'tcrminals of the channel ll by,means oi the leads II and thetransformer 20. In this case the input circuit oi the diode 23 isconnected by the leads 2I through a switch 220. which is operated bymeans of a cam II on the shaft ll so that the switch 221: closes oncefor each revolution of the support II. The compensating detector circuitand its manner of connection to the received signal amplifier is thesame as that of the arrangement of Figure 1.' 'In this case, however,the anode circuit of the triode I8 includes the signal operatedrecording lamp 200 in the recording head of the receiver.

In the operation oi this system, once for each revolution of thetransmitter the target 38 is interposed to cause excitation of thephotoelectrio device II at a uniform value to provide a reference signalwhich is amplified, sent over the transmission line, and applied to thecompensating detector 23. As before, the output of this detector isapplied to the received signal amplifier to bias it to a value whichwill compensate in this arrangement not only for any drift or variationin the signal output voltage of the photoelectric device I0, but for anyvariation or change in the ton line characteristics. As illustrated,this reference or compensating signal is preferably applied at the timethe scanning head and the transmitter would be scanning the clamping barl, which is also the time when the light from the recording lamp 200 isimpinged upon the clamping bar 4a of the receiver. Thus the visiblelight variation due to the compensating signal will not aiIect thesensitized negative 2'.

For the purpose oi orienting the reader's viewpoint, it may be noticedthat the systems of Figures 2 and 9 are quite similar in the equipmentused, and they really only dlifer in two main respects. Firstly, theswitch 22 01' the system of Figure 1 which has been designated switch22a 01 the system of Figure 9, has been moved from the transmitter tothe receiver. Secondly, the compensating detector has likewise beenshifted irom the transmitting end to the receiving end of the line. Thereceived signal amplifier is equivalent to the amplifier at thetransmitting end of the system oi Figure l but in actual use may notreally be the same amplifier because it is common to employ a signalamplifier at the receiving station so that in applying the system ofFigure 9 the biasing detector 23 and its associated circuits are simplyapplied to the receiving amplifier.

It will be apparent from the above description that the subject matterof this invention can be widely varied as suggested by the modificationsherein disclosed, and I do not, therefore, desire to bestrictly limitedto the disclosure as given for purposes 0! illustration. but rather tothe scope of the appended claims.

What is claimed is:

1. The combination with a facsimile picture transmitter having a seaminghead including 9.

- photoelectric device, and an amplifier fed by said device, of meansoperated periodically by said transmitter for applying a bias voltage tosaid amplifier to correct for the signal drift of said photoelectricdevice.

2. The combination with a facsimile picture transmitter having ascanning head and a transmission circuit connected to said scanninghead. oi means operated periodically by said transmitter for applying acompensating voltage to said transmission circuit at the transmittingend to compensate for inherent signal drlft therein.

3. In a system oi the type described the combination including afacsimile picture transmitter composed of a rotatable support for apicture to be transmitted, a scanning head including a light source anda photoelectric device, means for causing timed relative movementbetween said support and scanning head, a transmission network connectedto said photoelectric device, and means operated in timed relation withsaid support to apply a compensating voltage to said network at itstransmitting end once for each rotation of said support.

4. In an apparatus of the type described. the combination comprising amovable support for a picture to be transmitted. a reflecting membermoving in timed relation with said support. a scanning head including alight source and a photoelectric cell, means for causing a desiredrelative movement between said support and scanning head, a transmissioncircuit connected to said photoelectric cell, and means controlled bysaid relative movement for applying a biasing voltage to said network atits transmitting and to picture to be transmitted. a reflecting member,a

scanning head including a light source and a photoelectric cell, meansfor causing a desired relative movement between said suplm t,scanuingheadandreilectingmembenatrammissiorf circuit connected to saidphotoelectric cell. and means controlled by said relative movement forapplying a biasing voltage to said network to compensate ior inherentsignal drift in said network and in said photoelectric cell.

8. In a transmitting apparatus or the type described. the combinationcomprising a movable support for apicture to be transmitted, areiiecting member driven in timed relation with said a scanningheadincluding a light source and a photoelectric cell, means for causinga desired relative movement between said support and scanning head, atransmission circuit connected to said photoelectric cell. and vacuumtube repeating means including a network having a predetermined timeconstant operated as a result of said relative movement each time saidreflecting member is exposed to said light source to apply acompensating voltage to the network at the transmitting end tocompensate for inherent signal drift therein.

7. In a transmitting apparatus of the type described, the combinationcomprising a facsimile picture transmitter having a rotatable supportfor a picture to be transmitted, a reflective member synchronized withsaid support, a scanning head having a light source and a photoelectricdevice, means for causing relative longitudinal movement between saidsupport and scanning head as the support rotates. a transmission networkconnected to said photoelectric device, and meansconnected to saidtransmission circuit and operated each time said reflective member is ascanned by said light source to apply a compensating biasing voltage tothe network at its transmitting end.

B. In a facsimile picture ton system. the combination including arotatable support forapicturetobetransmitted.ascanning head including alight source, a photoelectric cell, and a movable reflective target,means for causing relative longitudinal movement between said scanninghead and support as the support rotates, and for causing said target tomove between said light source and support once for each rotationthereof, a on network connected to said photoelectric device, and meansenergised each time said target intercepts the lightiromsaidsourceiorapwi acompensating biasing voltage to said network at its transmittingend.

9. In a combination as disclosed, a facsimile Picture transmitterincluding a support for a scenetobetransmitted.ascanningheadhaving alight source and photoelectric device, means ior causing relativemovement between said scanningheadandsupportsoastosuccessivclrscanelementalareagoiasceneonsaidsupport. a transmission networkconnected to said photoelectric device, and mean connected to saidphotoelectric device and energised intermittently therebyduring'relative movcnent between saidsupportandscanningheadior applyingacompensating voltage to said network at its transmitting end.

10. In a combination as disclosed, a facsimile picture transmitterincluding a support for a scenetobetransmittemaheadhawlngalightsourc'eandphotoelactricdevicameans for causing relativemcvementbetween said scannin'gheadandsupportsoas'tosuccessivemscanelementalareaeciasceneonsaidsupport.

electric device. and means connected to said photoelectric device andenergised intermittently thereby during relative movement between saidsupport and scanning head for applying a compensating voltage to saidnetwork. said last means including an impedance network having a largetime constant,

11. In a combination as disclosed. a iacsimile picture transmitterincluding a support for a scene to be transmitted, a scanning headhaving a light source and photoelectric device, means for causingrelative movement between said scanning head and support so as tosuccessively scan elemental areas of a scene on said support, atransmission network connected to said photoelectric device, meansconnected to said photoelectric device and energized intermittentlythereby during relative movement between said support and scanning headfor applying .a coinpensating voltage to said network, said last meansincluding an impedance network having a large time constant, and meansfor applying a bucking voltage to said network to increase itsseniitivity.

12. In a combination as disclosed. a facsimile picture transmitterincluding a support for a scene to be transmitted, a scanning headhaving a light source and photoelectric device, means for causingrelative movement between said scanning head and support so as tosuccessively scan elemental areas oi a scene on said support. atransmission network connected to said photoelectric device, meansconnected to said photoelectric device and energized intermittentlythereby during relative movement between said support and scanning headfor applying a compensating voltage to said network, said last meansincluding an impedance network having a large time constant. a secondimpedance network having an equal-or a smaller time constant connectedto said first network, and means for applying a bucking voltage to saidsecond network.

13. In a system of the type described, a lacslmile picture transmitterincluding'a movable support for a scene to be transmitted. a scanninghead operating in conjunction therewith to produce picturesignals inaccordance with the variations in the tonal value of the scene, aiacsimile picture receiver including a support for a sensitized negativeon which the original scene istoberecordemareceiverincludinganamplii'ier l'or converting said signalsinto light variations and for applying them to the sensitised negative,a signal transmission network interconnecting the transmitter and thereceivm means operated by the transmitter for a 8acompensatingsignaltoaaidnetworkatte transmitting end at regularintervals.

14. In aiacsimilepicturetransmittlngandreceiving system, the combinationcomprising a facsimile picture transmitter including a scanning heademploying a photoelectric device, a

facsimile picturerecorder including arecording lamp, a transmissionnetwork interconnecting thetransmitterwiththerecorderandinchrdmgasignalamplifleratthetransmittingenmmeam for periodicallyeacitingsaidphotoelectrio device to produce a compensating signal, andmeans for intermittently applying said compensatingsignaltosaidampliilertobiastheampliflerscastocompensateiorsignalvoltagedrlitoithe photoelectric device.

KORBTANTIR WOIOBCHAK.

